1. Field of the Invention
The invention described herein relate generally to transceiver design and more specifically to the design of mixers for use in a plurality of transceivers.
2. Background
FIG. 1 illustrates an example device 100 that includes a transceiver 104 interfaced with a baseband processing circuit 102. The upper portion of transceiver 104 is the transmit portion and the lower portion of transceiver 104 is the receive portion. Transceiver 104 is configured to transmit and receive RF signals via antenna 132 using the transmit and receive portions of transceiver 104, respectively. The other end of transceiver 104 is interfaced with baseband processor 102.
For the transmit function, baseband processor 102 receives an input signal 110 and converts it into a digital baseband output signal 112. Baseband processor 102 then passes digital baseband output signal 112 to a Digital-to-Analog Converter (DAC), which is configured to convert output signal 112 into an analog baseband signal that can be upconverted, using upconversion mixer 122, to a RF signal capable of being transmitted via antenna 132. For example, baseband processor 102 can be configured to convert input signal 110 in accordance with the protocols and standards that govern the system in which device 100 is operating and the air interface being used.
A filter, such as filter 118 can be included to filter out unwanted noise and signals from the RF signal after conversion by mixer 122. In addition, a Power Amplifier (PA) 126 is typically included in the transmit portion of many transceivers. PA 126 is used to amplify the RF signal to a level that is sufficient to allow the signal to be transmitted over large distances. A second, bandpass filter 128 can also be included between PA 126 and antenna 132.
The term “baseband signal” is intended to refer to an information bearing signal that resides at DC, or Ø, in the frequency spectrum. In other words, a baseband signal can be said to comprise two components: an information, or data component; and a frequency component, where the frequency component is actually centered at Ø hertz (Hz). Thus, upconversion mixer 122 translates the frequency component of the input baseband signal from Ø Hz to a target, RF frequency, e.g., 900 MHz. But the data component is ideally unaffected by the translation.
The RF signal that results from the upconversion process performed by upconversion mixer 122 is often referred to as the carrier signal, and the RF frequency of the RF signal is often referred to as the carrier frequency, because the data is literally being carried by the RF signal to the destination when the RF signal is transmitted via antenna 132.
On the receive side, an RF signal is received by the receive portion of transceiver 104 via antenna 132. The received RF signal passes through a bandpass filter 130 and is then amplified by Low Noise Amplifier (LNA) 136. LNA 136 is configured to amplify the typically very low level received signal to a level that is sufficient for further handling, while adding very little noise to the amplified signal. The amplified signal is then converted via down conversion mixer 134 from an RF signal to an analog baseband signal. The analog baseband signal can be filtered using filter 128 and converted to a digital baseband input signal 116. Baseband input signal 116 is then converted to a digital baseband input signal 116 by Analog-to-Digital Converter (ADC) 108. The resulting digital baseband input signal 116 is then passed to baseband processor 102 so that it can process the data included in the baseband signal. Baseband processor 102 can be configured to then generate an output signal 112, such as a voice output signal, from the processed data.
Transceiver 104 is an example of a direct conversion transceiver because it converts the received RF signals directly to a baseband signal using mixer 134. Similarly, transceiver 104 is configured to convert baseband transmit signals directly to an RF signal using mixer 122. But it will understood that alternative transceiver architectures, e.g., Intermediate Frequency (IF) architectures, are also used in communication applications. All of these architectures have in common the need to translate signals from one frequency to another.
Mixers are well known circuits used to translate signals between different frequency bands. Frequency translation is implemented by multiplying the signal with a Local Oscillator (LO) signal, thereby translating the signal up or down to the new desired signal band. This type of translation is often referred to as modulation. For example, in transceiver 104 up conversion mixer 122 converts the baseband signal to an RF signal by modulating the baseband signal with an LO signal, which is generated, e.g., by a Voltage Controlled Oscillator (VCO) circuit 124. Mixer 122 combines the baseband and LO signals in accordance with the following equations:
                                                                                          V                  RFOUT                                =                                  A                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                              2                        *                        π                        *                        fo                                            )                                                        *                  B                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                              2                        *                        π                        *                                                  f                          LO                                                                    )                                                                                                                                              =                                                      (                                          1                      /                      2                                        )                                    *                  A                  *                  B                  ⁢                                                                          ⁢                                      cos                    ⁢                                                                                  [                                          2                      *                      π                      *                                              (                                                  fo                          ±                                                      f                            LO                                                                          )                                                              ]                                                                                      ⁢                                  ⁢                  f          RF                =                              f            0                    ±                                    f              LO                        .                                              (        1        )                            Where: ƒ0=the baseband center frequency;                    ƒLO=the LO center frequency; and            ƒRF=the target RF center frequency.                        
In other words, a mixer, e.g., mixer 122, combines the two input signals, i.e., the baseband signal and the LO signal, and produces two possible output signals: one characterized by the frequency (ƒ0+ƒLO); and one characterized by the frequency (ƒ0−ƒLO). The signal with the desired output is then selected, while the undesired signal is filtered out. For mixer 122, the signal characterized by the frequency (ƒ0+ƒLO) is selected while the signal characterized by the frequency (ƒ0−ƒLO) is filtered out, e.g., using filter 118. Thus, the output of mixer 122 is a signal characterized by the following frequency:ƒRF=(ƒ0+ƒLO)  (2).
In this case, ƒ0=Ø, because the signal is a baseband signal, which is defined as a DC or Ø frequency signal. Thus, ƒLO=ƒRF.
Conversely, the output of mixer 134 is a signal characterized by the following frequency:ƒ0=(ƒRF−ƒLO)  (3)
Broadband transceivers for many applications including, e.g., wireless communication applications, such as Wireless Local Area Networking (WLAN) applications, digital satellite applications, and cable TV applications, to name a few, all require an upconversion mixer on the transmit side to translate the frequency component of a baseband signal up to a carrier frequency in the Radio Frequency (RF) range, as well as a down conversion mixer 134 configured to translate received RF signals to baseband signals.
The design constraints are different for upconversion versus down conversion mixers, because the design constraints for the corresponding transmitters and receivers are different. For example, receivers must amplify signals spanning a wide dynamic range spanning several orders of magnitude of input signal power with low noise and minimum distortion. As a result, the noise figure and linearity must be optimized simultaneously to maximize the overall dynamic range. Conversely, the signal levels in a transmitter are typically large in amplitude. As a result, the design strategy on the transmit side is often focused on highest linearity performance together with lowest power consumption for longest battery life. Thus, the design of the upconversion and down conversion mixers, should take these design constraints into consideration as well.
Conventional mixers are often Gilbert Cell mixers. FIG. 2 illustrates a common Gilbert Cell topology that uses CMOS technology. The Gilbert Cell mixer 200 of FIG. 2 can be seen to comprise a transconductor 202 and a oscillator switching pair 204. Transconductor 202 comprises CMOS transistors M1 and M2. Oscillator switching pair 204 comprises CMOS transistors M3-M6. A bias current generator 206 is also included for biasing transconductor transistors M1 and M2. In mixer 200 all transistors M1-M6 are NFET devices.
For upconversion applications, the input transconductor converts a differential, baseband voltage signal (BBIN) into a baseband output signal current that is supplied to LO switching pair 204. Switching pair 204 is driven by a differential LO signal (LOIN) to modulate the baseband signal current. This modulated current is then converted to a voltage with the load resistors R1 and R2. The voltage output (VOUT) then comprises the baseband data signal (BBIN) translated up to the RF carrier frequency by the LO signal (LOIN).
Differential signals are used for their inherent rejection of common mode noise and for improved port to port isolation. For example, often a portion of the LO signal (LOIN) will “leak” into the BBIN input port. This LO leakage then mixes with itself, producing a DC offset on the output of the mixer. In a direct conversion receiver, for example, the DC offset produced by this self-mixing process can saturate the remaining stages of the receiver. Therefore, it is critical to minimize this LO to BB leakage using differential topologies.
For transmitters, the ability to eliminate unwanted noise and improve port to port isolation can be important, because, e.g., they often fall outside of the intended frequency spectrum, or channel, and into adjacent channels. The unwanted noise or leakage then interfere with devices operating on those adjacent channels. Such interference is referred to as adjacent channel interference, and how well a system or device handles such potential interference is referred to as the device's adjacent channel performance. A device's adjacent channel performance is often measured in terms of the adjacent channel power ratio (ACPR). The better a device's ACPR, the better the device's adjacent channel performance. Often, a device will be required to meet a certain minimum ACPR in order to comply with the operational requirements of a specific system.
A mixer can impact a device's adjacent channel performance if it is not designed well. Several criteria should be considered when designing a mixer in order to ensure good adjacent channel performance. These requirements include low LO signal leakage, good linearity, and low power consumption. Often, meeting the requirements for good adjacent channel performance requires a trade off among one or more of these criteria. The importance of low LO signal leakage was addressed above. Good linearity is important for adjacent channel performance because modulated signals passing through any weakly non-linear circuit broaden the bandwidth of the transmitted signal spectrum. This increased bandwidth, also called spectral re-growth, results in adjacent channel interference, because it encroaches on the bandwidth associated with the adjacent channels.
Low power is important because devices, such as device 100 are often powered by batteries, making power conservation imperative to sustain longer battery lifetimes. Moreover, applications that use double-balance mixers in highly integrated transceivers place additional emphasis on low power consumption. This is because the mixer often shares the same substrate with many other power-hungry RF blocks within the transceiver including power amplifiers, low noise amplifiers, variable gain amplifiers, and voltage controlled oscillators. Integrating all of these blocks on one die in the same package leads to a very stringent power budget.
With regard to LO leakage, the double-balanced Gilbert cell mixer of FIG. 2 is a fundamental topology that minimizes LO signal leakage, when the layout is optimized to ensure that the best possible matching is achieved. With regard to linearity, many topology variations of the standard double-balanced mixer exist. Often, the differences in these topologies are related to techniques for improving, or optimizing the linearity of the input transconductor.
For example, a widely used approach to achieve greater linearity is series feedback using emitter degeneration with respect the transconductor transistors M1 and M2. The degeneration can be in the form of a resistance or an inductance placed in series with the emitters, or in the CMOS case the sources, of transistors M1 and M2, although inductance is often only helpful for frequencies up to about 200 MHz. Emitter degeneration techniques offer good linearity and high 3rd order Intercept Point (IP3); however, such techniques also suffer several drawbacks including higher current, which results in higher power consumption, higher noise figure, and headroom problems at lower supply voltages. Also, degeneration is much more effective in technologies that use NPN devices as opposed to CMOS devices.
Another approach to the linearization problem in mixers is the multi-tanh technique. The advantage of the multi-tanh transconductor is higher linearity with smaller degradation in noise figure; however, the shortcomings of such designs include, higher current, similar to emitter degeneration, and a limited input linear range dependant upon the number of multi-tanh stages used. The extra current is necessary because two tail currents are needed to implement the piecewise linear transconductor.
Still another approach suggests using multiple gated MOSFETs by linear superposing several common source devices in parallel to improve linearity in mixers and low noise amplifiers (LNA). The main drawback to such techniques is that the DC biasing, critical for successful linearization, is complicated to achieve. Also, such techniques are specialized for down conversion in receivers and will not necessarily work for broadband, upconversion mixers.
Accordingly, conventional mixer designs present tradeoffs with respect to low power and high linearity requirements, and do not allow for optimization of both in the same design. As a result, there are many topologies for transconductors and mixers that achieve high linearity at the expense of increased power dissipation, or visa versa, but none that achieve optimal performance in all areas, namely low LO leakage, high linearity, and low power consumption.